What was the most “elegant” experiment ever? Many molecular biologists, who were active during the so-called “golden age” of the 1950s and 1960s, might opt for the 1958 experiment of Mathew Meselson and Franklin Stahl, which demonstrated the semiconservative replication of DNA (1). My choice is the 1960 experiment by Sidney Brenner, Francois Jacob, and Matt Meselson, which established the existence of messenger RNA (mRNA) (2). The story behind the discovery is an appropriate topic for the blog since bacteriophage T2 had a key role to play. It is told here, largely through the words of one of its contributors, Pasteur Institute scientist and Nobel laureate, Francois Jacob (3).

Imagine for the moment that we are back in the late 1950s, at a time when the precise role of RNA was not yet known. However, pertinent evidence was accumulating, which implied that RNA had a role in protein synthesis. For example, cellular RNA levels correlated with the levels of protein synthesis.

But what might the role of RNA be? The example of eukaryotic cells seemed to indicate that DNA could not directly serve as the template for protein synthesis. The DNA in those cells is contained within the membrane-bounded nucleus, whereas protein synthesis occurs in the cytoplasm. Might RNA then serve as an intermediate information carrier?

Jacob, and others, knew that protein synthesis took place in the cytoplasm, on tiny granules called ribosomes. Moreover, “for each gene there were corresponding ribosomes specifically charged with producing the corresponding protein (3).” This remark might seem to suggest an accurate view of protein synthesis. Nonetheless, the understanding of ribosomes at the time was fundamentally wrong. Each gene was thought to be transcribed to a unique RNA that became an integral component of a ribosome. Moreover, that integral RNA was thought to confer on the ribosome the specificity to support the synthesis of only the one protein that corresponded to that particular RNA—a scenario under which an entire ribosome needed to be produced de novo to support the translation of a gene.

With that view of ribosomes in mind, Jacob was troubled by the results from an earlier experiment, carried out in 1957 by Arthur Pardee, Jacob himself, and Jacques Monod—the famous (and also quite “elegant”) PaJaMa experiment (4). In this experiment, the Lac gene of an Hfr (male) strain of E. coli is transferred to a Lac-minus, F-minus (female) strain. [This experiment is famous because it was carried out under experimental conditions which enabled the three researchers to demonstrate the existence of a previously unknown regulatory molecule; the “repressor.”] What troubled Jacob was that the Lac gene of the donor E. coli strain was expressed “immediately upon entry of the gene…”—a result not in accord with the thinking of the day about the nature of ribosomes, and the way in which they translated genes into proteins.

It seemed impossible to Jacob that ribosomes, which are complex structures composed of proteins and RNA, could be produced quickly enough to enable the virtually immediate translation of the transferred Lac gene, as had been seen in the PaJaMa experiment. What’s more, the prevailing view of ribosomes also did not fit “with the existence of units of activity recently baptized ‘operons,’ that contained several genes. Nor with a regulation functioning directly on the DNA through the intermediary of a switch, now called an ‘operator.’”

The “perplexity prevailing in the Pasteur group” led to a new line of thought— “either direct synthesis of the protein on DNA itself, with no intermediary; or production of an unstable intermediary, probably an RNA with rapid renewal. But the former hypotheses seemed highly improbable and the latter without a chemical basis, without any trace of a molecule that could substantiate it.”

In 1959 Jacob attended a colloquium on microbial genetics in Copenhagen, where he intended to discuss this conundrum. “A small group attended, including notably Jim Watson, Francis Crick, Seymour Benzer, Sydney Brenner, Jacques (Monod), and even the physicist Niels Bohr. Courteous as ever, Jim Watson spent most of the sessions ostentatiously reading a newspaper. So, when it came time for him to speak, everyone took from his pocket a newspaper and began to read it”

When Jacob’s turn to speak came, he raised the possibility of a need for an unstable intermediary, which he called X. “No one reacted. No one batted an eyelash. No one asked a question. Jim continued to read his newspaper.”

“A new opportunity to discuss protein synthesis arose around Easter 1960 in Cambridge (England), in Sydney’s small apartment in King’s College, where he was a Fellow.” Although the meeting that morning was casual, several heavy hitters were present, including Francis Crick, Leslie Orgel, and Ole Maaloe, in addition to Jacob and Brenner.

Crick and Brenner discussed the results of a recent experiment carried out by Pardee and Monica Riley (Pardee’s student at the time). “They had succeeded in charging the DNA of male bacteria with radioactive phosphorus; in making them transfer to females the gene of galactosidase; in letting it synthesize the enzyme for some minutes; and then in destroying the gene through the disintegration of the radioactive phosphorus. The result was clear: once the gene was destroyed, all synthesis stopped. No gene, no enzyme. Which excluded any possibility of a stable intermediary.” [Recall the thinking that stable ribosomes contained an integral RNA that conferred its specificity.]

“At this precise point, Francis and Sydney leaped to their feet. Began to gesticulate. To argue at top speed in great agitation. A red-faced Francis. A Sydney with bristling eyebrows. The two talked at once, all but shouting. Each trying to anticipate the other. To explain to the other what had suddenly come to mind. All this at a clip that left my English far behind. For some minutes, it was impossible to follow them, just as it would have been impossible for them to follow a discussion in French between Jacques (Monod) and me. What had set off Francis and Sydney was, once again, a connection between the lactose system and phage. After infecting the colon bacillus, certain highly virulent phages blocked the synthesis of new ribosomes. As had been shown by two American Researchers, Elliot Volkin and Lazarus Astrachan, the only RNA then synthesized had two remarkable properties: on the one hand, unlike ribosomal RNA, it had the same base composition as DNA; on the other hand, it renewed itself very quickly. Exactly the properties required for what we called X, the unstable intermediary we had postulated for galactosidase. Why, in Paris, when we were looking for a support material for X, had we not thought of this phage RNA? Why had I not thought of it? Ignorance? Stupidity? Oversight? Misreading of the literature? Failure of judgment? A little of all these, no doubt. A mixture that, as in a detective novel, had made us fail to spot the murderer, the molecule responsible. In the last analysis, however, what mattered was that X, the unstable intermediary, was materializing…it had to be shown that all this was not a dream; that this RNA of the phage was indeed the unstable intermediary functioning in the synthesis of proteins: the issue that we and Sydney immediately decided to take up. …” See Aside 1.

[Aside 1: Volkin and Astrachan, at the Oak Ridge National Laboratory in Tennessee, showed that there actually are two kinds of RNA seen during phage infection—a stable type found in ribosomes (now known as ribosomal RNA, which does not have the same base composition as the DNA ), and an unstable, rapidly turning over type, that has the same base composition as the viral DNA, but not the bacterial DNA (5). Transfer RNA remained to be discovered.]

That afternoon, Jacob and Brenner found out that they each had been invited to spend a month (June) at the California Institute of Technology. Brenner’s invitation came from Matt Meselson, and Jacob’s from Max Delbruck. “A unique opportunity to work together to demonstrate the nature and role of X.” Importantly, Meselson recently developed a technique that would make the discovery possible.

That evening, at a party given by Crick and his wife, Jacob and Brenner discussed the experiment that they were envisioning. But: “It was difficult to isolate ourselves at such a brilliant, lively gathering, with all the people crowding around us, talking, shouting, laughing, singing, dancing. Nevertheless, squeezed up next to a little table as though on a desert island, we went on, in the rhythm of our own excitement, discussing our new model and the preparations for experiments at Caltech.”

In their new concept of protein synthesis: “The ribosomes had lost all specificity. They had become simple machines for assembling amino acids to form proteins of any kind, like tape recorders that can play any kind of music depending on the magnetic tape inserted in them. In protein synthesis, it was X, the unstable RNA copied on a gene, that had to play the role of the magnetic tape, associating with the ribosomes to dictate to them a particular sequence of amino acids corresponding to a particular protein.” Thus, the experiment would be to “show that the unstable RNA, synthesized after infection of a colon bacillus by the virulent phage, associated with pre-existing ribosomes, synthesized before infection, to produce the proteins of the phage.”

A key problem would be to distinguish ribosomes made before infection from any ribosomes that might be made after infection. Their solution would be provided by Matt Meselson’s new technique in which “he marked macromolecules by cultivating bacteria in heavy isotopes before putting them back in a normal environment. Using ultracentrifugation, he could then separate the marked molecules along gradients of density…”

Thus, the plan was to grow cells for several generations in medium containing the heavy isotopes 15N and 13C as the sole nitrogen and carbon sources, respectively. In this way, essentially all ribosomes present in the cells would be “heavy”. Next, the cells would be washed and placed in medium containing the normal isotopes, 14N and 12C. Then, the cells would immediately be infected with the phages. Any new ribosomes made after the infection got underway would be “light”.

Here is a key point. Recall that Volkin and Astrachan showed that the only RNA that is made after infection is the unstable RNA, which has the same base composition as the phage DNA. [That is so because the phage shuts down host transcription and translation.] Consequently, this phage RNA can be specifically labeled by adding 32P to the infected cultures (5). Brenner, Jacob, and Meselson hoped to find this rapidly turning-over phage-specific RNA in the density gradients, in association with the old heavy ribosomes that were made before infection. “If we were right, if our hypothesis was correct, the radioactivity of the RNA had to be associated, in the gradients, with the band of “heavy” ribosomes.”

However: “We were not succeeding.” The problem that was frustrating their efforts was that the ribosomes were unstable in the density gradients. “In vain did we try to check through the experiment, to modify it, to change a detail here and there. It was now three weeks since Sydney Brenner and I had arrived at the California Institute of Technology. We had come for the sole purpose of carrying out this experiment with Matt Meselson. An experiment that we had no doubt was going to change the world. But the gods were still against us. Nothing worked.”

“Our fine confidence at the start had evaporated. Disheartened, Meselson had departed-to get married! Sydney and I talked about going back to Europe. In a burst of compassion, a biologist by the name of Hildegaard had taken us under her wing and, to give us a change of scene, driven us to a nearby beach. There we were, collapsed on the sand, stranded in the sunlight like beached whales. My head felt empty. Frowning, knitting his heavy eyebrows, with a nasty look, Sydney gazed at the horizon without saying a word. Never yet had I seen Sydney Brenner in such a state. Never seen him silent…And our time was running out. For, come what may, Sydney and I had decided to leave at month’s end.”

“Hildegaard tried to tell us stories to lighten the atmosphere. But we were not listening. Suddenly, Sydney gives a shout. He leaps up, yelling, “The magnesium! It’s the magnesium!” Immediately we get back in Hildegaard’s car and race to the lab to run the experiment one last time. We then add a lot of magnesium… Sydney had been right. It was indeed the magnesium that gave the ribosomes their cohesion. But the usual quantities were insufficient in the density gradients used to separate heavy and light compounds. This time we added plenty of magnesium. The result was spectacular. Eyes glued to the Geiger counter, our throats tight, we tracked each successive figure as it came to take its place in exactly the order we had been expecting. And as the last sample was counted, a double shout of joy shook the basement at Caltech…This was merely one experiment, performed in extremis… But we now knew that we had won. That our conception explained the transfers of information in the synthesis of proteins…Scarcely was the experiment over than we gave a seminar at Caltech to demonstrate the existence of X and its role as magnetic tape. No one believed us. The next day we left, each to his own home. The bet had paid off. In the nick of time.”

Apropos our Virology blog, this experiment also showed that viruses subvert the cellular protein synthesis machinery for their own ends.

Addendum:

Nobel laureate Sidney Brenner was the main subject of two earlier posts—The Phage in the Letter, reposted September 8, 2016 and Sidney Brenner: Only Joking, January 5, 2014 (6, 7). Each of these posts highlighted Brenner’s mischievous sense of humor. Jacob offers more insight into Brenner’s personality in his account of the episode on the beach with Hildegaard: “There we were, collapsed on the sand, stranded in the sunlight like beached whales. My head felt empty. Frowning, knitting his heavy eyebrows, with a nasty look, Sydney gazed at the horizon without saying a word. Never yet had I seen Sydney Brenner in such a state. Never seen him silent. On the contrary, he was an indefatigable talker at every opportunity. A tireless storyteller, able to discourse for days and nights on end. Interminable monologues on every conceivable subject. Science, politics, philosophy, literature, anything that cropped up. With stories he made up as he went along. Generously laced with jokes. With nasty cracks, too, at the expense of just about everyone. An excellent actor, he could render a speech in Hungarian, a lecture in Japanese. Mimic Stalin or Franco. Even himself. He went without a break from one register to another. A sort of fireworks whose effects he gauged from the expressions of the people around him.”

In the September 8th reposting I wrote: “While Brenner’s work as a molecular biology pioneer may have justified a Nobel Prize, he received the award in 2002 for his later studies of the nematode Caenorhabditis elegans, in which his research group traced the fate of each cell from the zygote right through to the adult worm. Their work established C. Elegans as a model system that is now studied in hundreds of laboratories all over the world (6).”

Jacob collaborated with Jacques Monod to elucidate the genetic switch that regulates beta-galactosidase synthesis in E. coli. Their collaboration established the concepts of regulator genes and operons, for which they shared in the 1965 Nobel Prize for physiology or medicine.

François Jacob (left), with Jacques Monod and André Lwoff. This Pasteur Institute threesome shared the 1965 Nobel Prize for Physiology or Medicine “for their discoveries concerning genetic control of enzyme and virus synthesis.” Lwoff’s share of the award was for his pioneering studies of lysogeny (8).

In 1940, Jacob, who was Jewish, left medical school in occupied France to join Free French Forces in London. He then served as a medical officer in North Africa, where he was wounded, and was wounded again, this time severely, at Normandy in August 1944. Monod too was active in the French Resistance, during the Nazi occupation of Paris. He eventually become chief of staff of the French Forces of the Interior. In that capacity, he helped to prepare for the Allied landings in Normandy. Monod and Jacob each received France’s highest honors for their wartime service. For more on Jacob, see Genealogies and a Selective History of Lysogeny: Featuring Friedrich Loeffler, Emile Roux, Andre Lwoff, Elie Wollman, and Francois Jacob, posted January 28, 2015 (8).

Matt Meselson (still at Harvard at 86 years in age) is best known for showing that DNA replication is semi-conservative and for his part in the discovery of messenger RNA. Jacob tells us that at the time of their collaboration at Cal Tech: “He (Meselson) was haunted by the Cold War, by the need to establish better relations with the Soviet Union. In his soft voice, he could discourse for hours on strategy, tactics, nuclear arms, the Rand Corporation, first strikes, reprisals, annihilation.” Meselson later helped to persuade President Richard Nixon to renounce biological and chemical weapons, and to support an international treaty (the 1972 Biological Weapons Convention) banning the use of biological agents.

I am intrigued by the genealogies of our leading scientists, since their mentors too were often preeminent scientists. Earlier postings noted the example of Jonas Salk, who did postgraduate studies under Thomas Francis; one of the great pioneers of medical virology, perhaps best known for developing the first influenza vaccine (1, 2). James Watson, who did his doctoral studies in Salvatore Luria’s laboratory, and Renato Dulbecco, who trained under both Luria and Max Delbruck (3), are other examples. In fact, Watson and Dulbecco shared a lab bench in Luria’s lab. Howard Temin did his doctoral (and postdoctoral studies too) in Dulbecco’s lab (4). And Delbruck, who hugely influenced the new science of molecular biology, did his doctoral studies under Max Born, the 1954 Nobel Laureate in physics. Moreover, Delbruck later served as an assistant to Lisa Meitner (5).

Important research paths were undertaken, and major contributions were made, which resulted from less formal interactions between budding young scientists and top scientists of the day. Howard Temin’s chance encounter with Harry Rubin, while on a mission to Dulbecco’s lab, is a case in point (4).

Our last posting told how Louis Pasteur came within a whisker of adding the discovery of viruses to his list of extraordinary achievements (6). Robert Koch played a part in that story for developing his famous postulates, which provided the standard for demonstrating that a particular microbe causes a particular disease.

The Pasteur article also noted that in 1898 Friedrich Loeffler and Paul Frosch isolated the foot and mouth disease virus; the first virus isolated from animals. However, the piece did not point up that Loeffler had trained under Robert Koch. Also, it did not underscore the special significance of what Loeffler and Frosch achieved. In brief, by the 1890s Dmitry Ivanovsky and Martinus Beijerinck had independently discovered that the agent responsible for tobacco mosaic disease passes through bacterium-proof filters. Nevertheless, neither Ivanovsy nor Beijerinck appreciated the implication of their observation. Ivanovsky believed his filters might be defective, while Beijerinck thought the disease was caused by a “living liquid.” In contrast, Loeffler and Frosch, in addition to isolating the first virus that is pathogenic in animals, also carefully considered all possible explanations for their experimental findings, and then were the first to conclude the existence of a kind of microbe too small to be retained by bacterium-proof filters, and too small to be seen under a microscope, and that will not grow on laboratory culture media. They also correctly predicted that smallpox, cowpox, cattle plague, and measles are similarly caused by a “filterable virus.”

Loeffler made another major discovery, fourteen years earlier, in 1884, when he used his mentor’s postulates to identify the bacterium that causes diphtheria, Corynebacterium diphtheriae. Importantly, Loeffler also discovered that when he injected C. diphtheriae into animals, the microbe did not need to spread to the tissues it damaged. This observation led Loeffler to propose the bacteria were secreting a poison or toxin that spread to the remote sites and caused disease there.

Loeffler’s idea of a toxin was a new concept that subsequently was confirmed by Emile Roux, who had been Louis Pasteur’s assistant (6). Using bacterium-proof filters developed by Charles Chamberland in Pasteur’s lab, Roux showed that injecting animals with sterile filtrates of C. diphtheriae cultures caused death with a pathology characteristic of actual diphtheria. Roux was also a co-founder of the Pasteur Institute, where he was responsible for the production of diphtheria anti-toxin; the first effective diphtheria therapy. See Aside 1.

[Aside 1: Earlier, Roux suggested the approach Pasteur used to generate attenuated rabies virus for the Pasteur rabies vaccine (aging spinal cords from rabbits that succumbed to experimental rabies infections of their spinal cords). Roux later withdrew from the rabies project because of a disagreement with Pasteur over whether the rabies vaccine might be safe for use in humans (6).]

So, Loeffler and Roux trained under Koch and Pasteur, respectively. But why might toxin production by C. diphtheriae interest virologists. Well, in 1951, Victor Freeman at the University of Washington showed that the lethal toxins produced by C. diphtheriae (and by Clostridium botulinum as well) are the products of lysogenic bacteriophage carried by the bacteria. This was shown by the finding that avirulent strains of these bacteria became virulent when infected with phages that could be induced from virulent strains. So, are diphtheria and botulism due to bacteria or to viruses? Our chain of genealogies continues with a selective history of lysogeny.

Almost from the beginning of phage research (bacteriophage were discovered independently by Frederick Twort in Great Britain in 1915 and by Félix d’Hérelle in France in 1917), some seemingly normal bacterial cultures were observed to generate phage. Initially, this phenomenon was thought to be a sign of a smoldering, steady state kind of persistent phage infection. Then, during the 1920s and 1930s, the French bacteriologists, Eugene Wollman and his wife Elizabeth, working together on Bacillus megatherium at the Pasteur Institute, provided evidence that instead of a steady state infection, the phage actually enter into a latent form in their host cells; a form in which they might be harmlessly passed from one cell generation to the next. [Considering the state of knowledge back then, note the insightfulness of Eugene Wollman’s 1928 comment, “the two notions of heredity and infection which seemed so completely distinct and in some ways incompatible, . . . almost merge under certain conditions.”] See Aside 2.

[Aside 2: Since some bacterial strains would, on occasion, spontaneously undergo lysis and release bacteriophage, the cryptic bacteriophage they carried were called “lysogenic.” Thus, it is a bit odd that “lysogeny” eventually came to refer to the temperate relationship between these phages and their host cells.]

In the late 1930s, the Wollmans developed a close friendship with Andre Lwoff, their new colleague at the Pasteur Institute. The Wollmans introduced Lwoff to their ideas about lysogeny, but, as Lwoff confesses, he was not then impressed by bacteriophage (7).

The Nazi occupation of Paris during the Second World War began in 1940. From then on, the Jewish Wollmans were prevented from publishing their research findings. Nevertheless, they continued their research at the Pasteur Institute until 1943, when they were seized by the Nazis and sent to Auschwitz. They never were heard from again. Their friend, Lwoff, grieved their loss and became active in the French resistance, gathering intelligence for the Allies, while also hiding downed American airmen in his apartment.

After the war, Lwoff received several honors from the French government for his efforts against the Nazis. He also returned to his research at the Pasteur Institute, studying the genetics of Moraxella; a bacterial pathogen of the human respiratory tract. Because of his work as a microbial geneticist, he was invited to the 1946 Cold Spring Harbor Symposium, where he met Max Delbruck. And as happened to others, meeting Delbruck resulted in Lwoff being seduced by bacteriophage.

Andre Lwoff

Back in Paris, Lwoff’s passionate interest in phages was heightened further by discussions with Jacques Monod, a friend of Max Delbruck, and Lwoff’s neighbor in the attic of the Pasteur Institute. Although Monod was Lwoff’s junior colleague (in fact, it was Lwoff who first stirred Monod’s interest in microbiology), Lwoff’s conversations with the future Nobel Laureate resulted in Lwoff becoming intensely fascinated by lysogeny, which he began to study in 1949 (7).

Because of Lwoff’s earlier friendship with the Wollmans, he chose to study a lysogenic strain of B. megatarium. And, making use of techniques he learned from Renato Dulbecco during a brief stint at Cal Tech, he was able to follow a single lysogenic bacterium, which enabled him to observe that a bacterium could go through multiple rounds of replication without liberating virus. What’s more, he discovered that the phages are released in a burst when the cell lyses, thereby dispelling the still current notion that phages are liberated continuously by lysogenic bacteria. Furthermore, Lwoff showed that lysogenic bacteria usually do not contain phage particles, since none are detected when the cells are experimentally lysed with lysozyme; confirming the earlier (1937) findings of the Wollmans.

Lwoff went on to show that temperate phage genomes are maintained in a previously unknown integrated state in their host cell, and he gave the integrated phage genomes a name, “prophage.” He also discovered, unexpectedly, that irradiating lysogenic bacteria with ultraviolet light could induce the temperate phages to emerge from their latent state, and then replicate in, and lyse their host cells. And, he discovered that the phages lyse their host bacterial cells by producing enzymes that destroy bacterial cell walls.

Prophage

Lwoff’s elucidation of the fundamental nature of lysogeny in bacteria would later provide a paradigm for the DNA tumor viruses, the herpesviruses, the oncogenic retroviruses, and HIV. He was awarded a share of the 1965 Nobel Prize for physiology or medicine for his lysogeny research. He shared the award with his fellow Pasteur Institute scientists, François Jacob and Jacques Monod, who received their awards for their pioneering studies of gene regulation in E. Coli.

A rather intriguing aspect of this story is that Lwoff was joined in his research on lysogeny at the Pasteur Institute by Elie Wollman; the son of Eugene and Elizabeth. Elie, born in 1917, escaped from the Nazis in Paris in 1940 and worked in the French resistance as a physician. In 1946, after the war, he came to the Pasteur Institute, where he took its microbiology course and then became Lwoff’s research assistant. Then, in 1947, Elie too happened to meet Max Delbruck (in Paris in this instance) and was invited to join the Cal Tech phage group, where he spent the next two years. See Aside 3.

Elie Wollman

[Aside 3: By the early 1940s, the then young Cal Tech “phage group,” headed by Max Delbruck, was on its way to becoming the World’s great center for phage research (5). However, the American group had little interest in lysogeny, since Delbrück neither believed in it, nor saw its importance. Instead, Delbruck was totally committed to the study of lytic phages. Then, during the late 1940s, Delbruck began to lose interest in molecular biology and looked for new research directions. When he thought of turning his attention to brain function, he asked his group to put together a series of seminars based on papers written by prominent neuroscientists of the day. Elie Wollman was the only member of the Cal Tech group who declined to participate in that endeavor, since he was totally committed to bacteriophage. Moreover, Elie was the one who finally convinced Delbruck that “such a thing as lysogeny does exist (7).”

Elie himself tells us that when he looked into a bibliographical index at Cal Tech, he came across an index card referring to his parent’s 1937 paper, which reported their finding that lysogenic cells contain a non-infectious form of the phage (8). “Delbruck’s comment on the card was “Nonsense.”]

After Eli’s two-year stint with Delbruck in Pasadena, he returned to the Pasteur Institute. Meanwhile, Francois Jacob had come to the Institute in the hope of beginning a research career in genetics under the tutelage of either Lwoff or Monod. Before that, in 1940, Jacob, who also was Jewish, left medical school in occupied France to join Free French Forces in London. He then served as a medical officer in North Africa, where he was wounded, and was later severely wounded at Normandy in August 1944, ending his dream of becoming a surgeon.

Francois Jacob

Initially, Jacob was spurned by both Lwoff and Monod, but was finally taken on by Lwoff, who suggested that he, Jacob, start work on “the induction of the prophage.” Jacob confesses he had no idea what that meant, but he accepted the project. Thus it came to pass that Francois Jacob and Elie Wollman established a particularly close and friendly collaboration, in which they turned their attention to the lambda prophage of E. coli. Their initial goal was to clarify the events of bacterial conjugation so that they might then understand the phenomenon whereby a temperate phage carried by a lysogenic bacterium is activated to undergo vegetative replication when that bacterium conjugates with, and transfers its integrated phage genome to a non-lysogenic bacterium.

To accomplish their goal, Wollman and Jacob began with experiments to locate the lambda genome on the chromosome of the lysogenic cell, and to follow its transfer during conjugation into a non-lysogenic recipient cell. A key feature of their experimental approach was conceived by Wollman (8). It was simply to interrupt conjugation between a lysogenic donor (Hfr) cell and a non-lysogenic recipient (F-minus) cell, at various times, by using a kitchen blender to break the mating cells apart. Using the blender to interrupt conjugation, and also using bacterial strains in which the recipient bacteria contained a set of mutations, and plating the mating mixture on selective media, Wollman and Jacob were able to measure the length of time required for each of the corresponding wild-type genes to be transferred from the Hfr donor cells to the F-minus recipient cells. Indeed, the time intervals between the appearances of each wild type gene in the recipient cells directly correlated with the distances between the genes, as independently determined by recombination frequencies. Thus, the interrupted mating approach gave Wollman and Jacob a new means to construct a genetic map of the bacterium, while also enabling them to locate the integrated phage genome on that map. Their experimental approach also allowed Wollman and Jacob to establish that, during conjugation, the donor cell’s genome is transferred linearly to the recipient cell. [The designation “Hfr” was coined by William Hayes because Hfr strains yielded a high frequency of recombinants when crossed with female strains.]

Importantly, Wollman and Jacob’s study of the activation of a lambda prophage when it enters a non-lysogenic F-minus recipient (a phenomenon they called “zygotic induction”), showed that the temperate state of the lambda prophage is maintained by some regulatory factor present in the cytoplasm of a lysogenic bacterium, but which is absent from a non-lysogenic one. It led to the discovery of a “genetic switch” that regulates the activation of the lysogenic bacteriophage, and of a phage-encoded repressor that controls the switch. These findings are among the first examples of gene regulation, and are credited with generating concepts such as the repressor/operator, which were firmed up by Jacob and Monod in their Nobel Prize-winning studies of the E. coli lac operon. See Aside 4.

[Aside 4: At the time of Wollman and Jacob’s interrupted mating experiments, kitchen blenders had not yet made their way to European stores. Eli was aware of these appliances only because of his earlier stint at Cal Tech. He bought a blender for his wife before returning to France, and then “borrowed” it for these experiments.]

Wollman and Jacob went on to demonstrate that the fertility or F factor, which confers maleness on the donor bacteria, can exist either in an integrated or an autonomous state. Indeed, this was the first description of such a genetic element, for which they coined the term “episome;” a term now largely replaced by “plasmid.”

Wollman and Jacob also determined that the E. coli chromosome is actually a closed circle. The background was as follows. Only one F factor is integrated into the chromosome of each Hfr strain, and that integration occurs at random. And, since the integrated F factor is the origin of the gene transfer process from the Hfr cell to the F-minus cell, interrupted mating experiments with different Hfr strains gave rise to maps with different times of entry for each gene. However, when these time-of-entry maps were taken together, their overlapping regions gave rise to a consistent circular map. The discovery of the circular E. coli chromosome was most intriguing, because all previously known genetic maps were linear. See Aside 5.

[Aside 5: The bacterial strain used by Wollman and Jacob in their study of zygotic induction was, in fact, the original laboratory strain of E. coli (i.e. E. coli K12) that was isolated in1922 from a patient with an intestinal disorder. In 1951, Esther Lederberg discovered that K12 is lysogenic. The discovery happened when she accidentally isolated non-lysogenic or “cured” derivatives of E. coli K12 that could be infected by samples of culture fluid from the parental K12 strain, which sporadically produced low levels of phage. Esther gave the lysogenic phage its name, lambda.

Esther was the wife of Joshua Lederberg, who received a Nobel Prize in 1958 for discovering sexual conjugation in bacteria, and the genetic recombination that might then ensue. Prior to Lederberg’s discoveries, genetic exchange and recombination were not believed to occur in bacteria. Lederberg’s Nobel award was shared with George Beadle and Edward Tatum (the latter was Lederberg’s postdoctoral mentor) for their work in genetics.

Joshua Lederberg, working with Norton Zinder (9), also discovered transduction, whereby a bacterial gene can be transferred from one bacterium to another by means of a bacteriophage vector. And, working together with Esther, Joshua discovered specialized transduction, whereby lambda phage transduces only those bacterial gene sequences in the vicinity of its integration site on its host chromosome. Esther and Joshua also worked together to develop the technique of replica plating, which enabled the selection of bacterial mutants from among hundreds of bacterial colonies on a plate and, more importantly perhaps, to provide direct proof of the spontaneous origin of mutants that have a selective advantage.]

In 1954 Elie Wollman was appointed a laboratory head in his own right at the Pasteur Institute. He retired from research in 1966 to become vice-director of the Institute, which he then rescued from a severe financial crisis. He continued to serve in that role for the next 20 years, while garnering numerous prestigious awards for his research and service.

Francois Jacob earned his doctorate in 1954 for his lysogeny studies. Then, realizing that he and Jacques Monod, his senior neighbor in the Pasteur Institute attic, were actually studying the same phenomenon, gene repression, he entered into a hugely productive collaboration with Monod that led to the elucidation of the genetic switch that regulates beta-galactosidase synthesis in E. coli (9). Their collaboration established the concepts of regulator genes, operons, and messenger RNA, for which they shared in the 1965 Nobel Prize for physiology or medicine, as noted above. See Asides 6 and 7.

Jacques Monod

[Aside 6: One of Jacob and Monod’s first experiments was the famous 1957 PaJaMa experiment, carried out in collaboration with Arthur Pardee, who was then on sabbatical at the Pasteur Institute. In brief (for aficionados), a Lac-positive, Hfr strain was grown in an inducer-free media, and then mated, still in an inducer-free media, with a Lac-minus, F-minus strain. (Note that the deletion in the Lac-minus, F-minus strain included the LacI gene, which encodes the yet to be discovered lac repressor.) As expected, in the absence of inducer, no beta-galactosidase is detected initially. But, after the donor DNA sequence, which bears the normal Lac genes (including LacI), is transferred to the Lac-minus recipient, it initially finds no repressor in the recipient cell and begins to synthesize beta-galactosidase. Then, as the donor cell’s lac repressor gene begins to be expressed in the recipient cell, in the inducer-free media, expression of the donor cell’s beta-galactosidase gene ceases. The PaJaMa experiment thus showed that the genetic regulation of enzymatic induction depends on a previously unknown regulatory molecule, the repressor.

Notice the similarity between the rationale for the PaJaMa experiment and that of the earlier Wollman and Jacob experiment on zygotic induction. In each instance, a process regulated by a repressor is suddenly in the repressor-free environment of a recipient cell.]

[Aside 7: In June of 1960, Francois Jacob, Matt Meslson, and Sidney Brenner came together in Max Delbruck’s Cal Tech lab to carry out an experiment that confirmed the existence of messenger RNA. The key to the experiment was their ability to distinguish ribosomes present in the cell before infection from ribosomes that might have been made after infection. They cleverly did that by incorporating heavy isotopes into ribosomes before infection, so that they might be separated in a density gradient from ribosome made after infection. Then, they showed that RNA produced by T2 phage in E. Coli associates with ribosomes that were synthesized by the cell entirely before infection. Furthermore, the new phage-specific RNA directs the synthesis of phage-specific proteins on those “old” ribosomes. I vote for this experiment as the most elegant in the entire history of molecular biology (11).]

Incidentally, during the Nazi occupation of Paris, Monod too was active in the French Resistance, eventually becoming chief of staff of the French Forces of the Interior. In that capacity, he helped to prepare for the Allied landings in Normandy. Monod and Jacob each received France’s highest honors for their wartime service.See Aside 7.

[Aside 7: I am singularly intrigued by the experiences of Andre Lwoff, Elie Wollman, Francois Jacob, and Jacques Monod during the Second World War. References 3 and 5 recount the wartime experiences of Renato Dulbecco and of Max Delbruck, and of other great scientists of the time. Other posts on the blog give accounts of virologists courageously placing themselves in harm’s way under different circumstances. Examples include pieces featuring Ciro de Quadros, Carlo Urbani, Peter Piot, and Walter Reed.]

References:

(1) Jonas Salk and Albert Sabin: One of the Great Rivalries of Medical Science, Posted on the blog March 27, 2014.

(2) Ernest Goodpasture and the Egg in the Flu Vaccine, Posted on the blog November 25, 2014.

(3) Renato Dulbecco and the Beginnings of Quantitative Animal Virology, Posted on the blog, December 4, 2013.

(4) Howard Temin: “In from the Cold,” Posted on the blog December 16, 2013
(5) Max Delbruck, Lisa Meitner, Niels Bohr, and the Nazis, Posted on the blog November 12, 2013.

(6) Louis Pasteur: One Step Away from Discovering Viruses, Posted on the blog January 7, 2015.

Blogs I Follow

Welcome!

I am now a retired professor emeritus of Microbiology at the University of Massachusetts. Teaching virology has been a most rewarding aspect of my career. I especially enjoyed enlivening my lectures with a variety of relevant anecdotes.

Virology Textbook

Based on my experiences teaching virology for more than 35 years, I wrote Virology: Molecular Biology and Pathogenesis (ASM Press; 2010). For info on adopting or buying this textbook, please visit the publisher site: http://www.asmscience.org/content/book/10.1128/9781555814533